This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A352121 #15 Mar 06 2022 08:40:58
%S A352121 1,1,-3,13,-87,841,-10683,167413,-3113967,66991441,-1635760563,
%T A352121 44683635613,-1350018280647,44694643670041,-1608962582321643,
%U A352121 62572776778020613,-2614314267900284127,116781203402752052641,-5553985490569476301923
%N A352121 Expansion of e.g.f. sqrt(2 - exp(-2*x)).
%F A352121 a(n) = Sum_{k=0..n} (-2)^(n-k) * (Product_{j=0..k-1} (-2*j+1)) * Stirling2(n,k).
%F A352121 a(n) ~ (-1)^(n+1) * 2^n * n^(n-1) / (log(2)^(n - 1/2) * exp(n)). - _Vaclav Kotesovec_, Mar 06 2022
%t A352121 m = 18; Range[0, m]! * CoefficientList[Series[(2 - Exp[-2*x])^(1/2), {x, 0, m}], x] (* _Amiram Eldar_, Mar 05 2022 *)
%o A352121 (PARI) my(N=20, x='x+O('x^N)); Vec(serlaplace(sqrt(2-exp(-2*x))))
%o A352121 (PARI) a(n) = sum(k=0, n, (-2)^(n-k)*prod(j=0, k-1, -2*j+1)*stirling(n, k, 2));
%Y A352121 Cf. A352122, A352123.
%Y A352121 Cf. A352075.
%K A352121 sign
%O A352121 0,3
%A A352121 _Seiichi Manyama_, Mar 05 2022